In recent years, there has been a tendency that, with the increase in the density and accuracy of ultra-LSI devices, the requirement for an increase in the fineness and flatness of a substrate surface required for a mask blank substrate, a mask blank, a transfer mask, or the like for use in the manufacture of ultra-LSI devices has become severe more and more year by year. In particular, as the wavelength of an exposure light source has been shortened, the requirement for the shape accuracy (flatness) and quality (defect size) of a substrate surface has become severe more and more, and accordingly, there has been required a mask blank substrate or the like with extremely high flatness and with no microdefects. In the case of a mask blank substrate for use in a reflective mask adapted to reflect exposure light, the requirement for the shape accuracy and quality of a substrate surface is particularly severe.
In order to respond to the above-mentioned requirement, various techniques have been developed.
For example, Patent Document 1 discloses a mask blank glass substrate manufacturing method which comprises a polishing process of polishing a surface of a mask blank glass substrate (also called a mask blank substrate) using a polishing liquid containing polishing abrasive particles.
This technique is characterized in that the polishing abrasive particles contain colloidal silica abrasive particles produced by hydrolyzing an organic silicon compound and that the polishing liquid contains the colloidal silica abrasive particles and is neutral.
Patent Document 2 discloses a technique for a mask blank glass substrate manufacturing method which comprises a concave-convex shape measurement process of measuring the concave-convex shape of a surface of a mask blank glass substrate, a flatness control process of specifying the convex degree of a convex portion present on the glass substrate surface based on measurement results obtained in the concave-convex shape measurement process and applying local processing to the convex portion under processing conditions according to the convex degree to thereby control the flatness of the glass substrate surface to a predetermined reference value or less, and a polishing process, after the flatness control process, of polishing the glass substrate surface subjected to the local processing. This technique is a method that applies an acid treatment to the glass substrate surface subjected to the local processing, after the flatness control process and before the polishing process.
The technique of the Patent Document 2 comprises the concave-convex shape measurement process, the flatness control process, and so on and comprises a preparation process before the concave-convex shape measurement process.
This preparation process comprises at least a rough polishing process of roughly polishing both surfaces of the mask blank substrate and a precision polishing process of precisely polishing both surfaces of the mask blank substrate roughly polished, thereby carrying out polishing stepwise. In this event, for example, the rough polishing process uses a polishing liquid containing relatively large polishing abrasive particles of cerium oxide while the precision polishing process uses a polishing liquid containing relatively small polishing abrasive particles of colloidal silica.
On the other hand, conventionally, in a mask blank substrate, in order to make it possible to judge the kind of substrate or distinguish between the front and back of the substrate, use has been made of a substrate mark (or also called a notch mark) formed by cutting a main surface into the shape of an oblique section at a corner (also called a corner) of the rectangular mask blank substrate.
Various techniques related to the above-mentioned substrate mark have been developed.
For example, Patent Document 3 discloses a technique for a mask blank transparent substrate which is required to have predetermined optical properties and is characterized by having a substrate mark formed by cutting a predetermined corner portion into the shape of an oblique section, wherein this shape is determined according to the optical properties.
Patent Document 4 discloses a technique for a photomask substrate with a generally rectangular shape, which is characterized by having at least one notch mark formed by cutting, at a corner of such a rectangular shape, three surfaces, i.e. a main surface and two end faces forming such a corner, into the shape of an oblique section, wherein the notch mark has an asymmetrical shape with respect to a diagonal, including such a corner, of the photomask substrate.